A conventional terminal structure for an optical device includes a short-length optical fiber of up to several tens mm fixed in a ferrule having an inside diameter slightly larger than a diameter of optical fibers to be connected through the terminal structure. The short-length optical fiber has a normalized frequency V in operation state of approximately 2.7 at a wavelength of 1.3 .mu.m which is equivalent to a single mode fiber with no bending loss used as a long-length optical transmission path.
The normalized frequency V is defined by the following formula: EQU V=2.pi.a/.lambda..multidot.(n.sub.1.sup.2 -n.sub.2.sup.2).sup.1/2( 1)
or the approximate formula thereof: EQU V=2.pi.a/.lambda..multidot.(2.DELTA.).sup.1/2 ( 2)
where a is a core radius of an optical fiber, .lambda. is a wavelength of a core light used as a signal carrier, n.sub.1 and n.sub.2 are refractive indexes of a core and a clad of the optical fiber respectively, and .DELTA. is defined as (n.sub.1 -n.sub.2)/n.sub.1.
As understood by the formulas, the normalized frequency V is inversely proportional to the wavelength .lambda., directly proportional to the radius a, and approximately proportional to square root of the relative difference between the two refractive indexes, that is .DELTA.. The transmission light has a single mode when the normalized frequency V in operation state is under 2.405, and has LP.sub.01 and LP.sub.11 modes (linearly polarized modes 01 and 11) when the normalized frequency V is equal to or over 2.405 and under 3.83.
According to the conventional terminal structure for an optical device, however, there is a disadvantage in that there is a periodical characteristic of wavelength dependency in the transmission loss. The short-length optical fiber of the terminal structure has two misaligned joints with the connected optical fibers, LP.sub.01 mode which is a transmission mode already existing in the optical fiber devices into LP.sub.11 mode and LP.sub.01 mode at the first misaligned joint. The two modes are propagated in the short-length optical fiber, and are divided LP.sub.01 mode and LP.sub.11 mode again at the second misaligned joint LP.sub.01 mode which is divided from LP.sub.11 mode and the LP.sub.01 mode which is propergated through the short length fiber interferes each other. Therefore, the connection efficiency at the second misaligned joint has the periodical characteristic of wavelength dependency in the transmission loss. Such a wavelength periodical dependency in the transmission loss may cause changes of the transmission loss by a slight change of a wavelength in use. Further, the change of the transmission loss is affected by a misaligned degree of the joints or direction change thereof. Consequently, a slight change of the wavelength or phase of the light source is converted to a change of a light intensity by the terminal structure, so that noises are generated in the light signal.